High density optical recording systems which may be used for recording and playing back information are known in the prior art. For example, U.S. Pat. No. 4,097,895, entitled "Multi-Layer Optical Record," issued on June 27, 1978, to F. W. Spong, relates to an optical disc record/playback system wherein data are recorded on the surface of a recording medium. In a Spong system the thermal energy of a focused high intensity light beam causes variation in the optical properties on the surface of the recording medium. For example, in one system the thermal effects of a laser beam form pits in an absorptive coating on the surface of an optical disc. In the Spong system, approximately 10.sup.11 bits of information can be recorded on one side of a disc-shaped record medium having a thirty centimeter diameter.
Optical mass data storage systems have developed to the point where they are finding increased application to information processing systems. The advantages of the optical techniques lie in their packing density and the increased data rates with random access retrieval. The potential for improvement in optical data storage will most likely be directed toward the record medium. Although acceptable archival materials for permanent data storage have been developed, research continues to find more efficient techniques and materials to increase packing density, signal-to-noise ratio, and data rates. Nevertheless, in order for optical storage systems to provide total competition for magnetic storage systems, it is necessary to provide an efficient erasable and reusable medium, wherein the erasing procedure is executed in a time period comparable to that required for data recording.
Currently, magneto-optic materials are used in the state-of-the-art recording media to effect readily alterable recordings. A magneto-optic recording medium is a magnetic material which causes the polarization angle of laser light to be changed when reflected from a recorded spot. Preparatory to recording or playback operation, the molecules of the magneto-optic material across the entire recording surface are vertically oriented in one direction. During recording, the molecules of the magneto-optic material at the point of incidence of a laser beam are flipped in the opposite direction due to the presence of a magnetic field of a given strength and polarity and the heating induced by the laser. Illumination from a playback laser beam of lower power reflected from this area will show a polarization angle change. The erasing process then restores the molecular orientation condition which existed prior to recording by the use of a magnetic field of polarity opposite to that used for recording in the presence of a continuous wave laser beam.
Many optical disk storage systems which use the magneto-optic technique have preformatted pilot tracks on the recording surfaces of the disks. That is, each disk has a convolution of permanently recorded information which forms a single spiral track from a point near the outer circumference of the disk to a point near the disk central aperture. The information recorded on this pilot track may typically include data useful in clocking and in determining the track position, e.g., radius and sector. The method by which this information is permanently recorded in the disk surface may typically use an ablative process similar to that described in the Spong patent referenced earlier. With the optical head following the preformatted pilot track under closed loop serve control, data tracks can be recorded on, and played back from, the disc information surface adjacent the pilot track, using a reversible recording process.
There are, however, magneto-optic disk recording systems which use blank, unformatted disk media. The unformatted disk is cheaper, far simpler to fabricate, and offers more flexibility to the user. The unformatted disk does, however, present a tracking problem during erasure. Typically, during the recordation process, the recorded data are checked by a read-after-write process in which the data are read immediately following the write operation. Since data erasure on a magneto-optic disk is substantially the same process as data recordation, data on a preformatted disk may be erased by virtually the same elements used for recording data, while following the pilot track. Since tracking on an unformatted disk is accomplished by reading recorded data, the erasure process on this type of disk requires the ability to read data ahead of the write/erase beam. The effect is to require a system having the capability of reading data following the write beam but preceding the erase beam.